As a result of continuing global population and income growth, electricity demand continues to increase worldwide. Electricity grids are required to adjust to large somewhat unpredictable swings in demand as well as the planned and unplanned changes in production capacity. Additionally, alternative sources such as wind and solar generated power are growing in importance and these sources have an impact in the way that power is generated to meet the demand.
Electricity demand is chaotic. Demand may vary on a daily, monthly, seasonal and yearly cycles. For example, a typical residential daily demand profile on a hot day may show a minimum in the early morning hours, and a maximum in the early evening hours. Commercial demand on the same day may show a minimum in the early evening hours and a maximum around the middle of the day. Weather and season of the year also impact demand. The peak demand may in some cases be double the minimum demand.
Because electricity generated by power companies cannot be efficiently stored, electric utilities have traditionally generated power with a combination of different approaches to production. For example, large nuclear or coal fired plants may be used for generating a minimum amount of power (baseload). Baseload power plants typically operate continuously at maximum output.
During times of peak demand (peak load) power companies may use simple cycle gas turbines for generating power. Gas turbines are desirable for supplying the additional capacity required during peak loads because of their ability to start up quickly, producing electrical power in 10 to 30 minutes. Gas turbines used to generate power during periods of peak loads may be shut down for portions of a day when the demand for power is low. The period of operation of the gas turbines may vary in accordance with the demand.
Some utilities also operate load following plants that run during the day to supply power during periods of intermediate demand. Combined cycle gas turbine systems are sometimes used for as load following plants. COMBINED CYCLE GAS TURBINE systems typically include a heat recovery steam generator coupled to the exhaust of the gas turbine. combined cycle gas turbine systems may adjust their power output as demand fluctuates throughout the day. combined cycle gas turbine systems are typically in between base load power plants and peaking plants (e.g. gas turbines used to provide peak power) in efficiency, speed of startup and shutdown, and capacity.
To meet the increased demands and address environmental concerns, many utilities are using sources of renewable energy, such as wind and solar power to meet intermediate and peaking loads. These sources add additional variability to the electricity demand because of their intermittent generation capacity. For example, power output of a solar electricity generation plant varies depending on the cloud cover and, similarly wind power output will vary depending on wind speed.
Gas turbines have a number of advantages as sources of power for peak loads. Gas turbines are efficient, have a relatively low installed cost, have a relatively fast start up, and shut down and low emissions. The startup sequence of a gas turbine begins with energizing a starter. When the RPM of the turbine reaches a light up RPM the ignition systems are energized and fuel is provided to the combustor. Upon combustion, the fuel flow is increased while maintaining temperatures below established temperature limits. Fuel flow is then controlled to achieve smooth acceleration until idle speed is reached.
A gas turbine may be operated at base load, peak load, and loads below the base load. The gas turbine baseload is the load that optimizes power output, and hot gas path parts life. ANSI B133.6 Ratings and Performance defines base load as operation at 8,000 hours per year with 800 hours per start. It also defines peak load as operation at 1250 hours per year with five hours per start. The peak load of a gas turbine is a load that maximizes power output, frequently at the expense of efficiency, parts life and inspection intervals. Gas turbines may be operated at partial or low loads in order to be able to quickly ramp up to higher output when demand for power increases. There are advantages and disadvantages in operating a gas turbine at partial loads. One advantage is to reduce the plant maintenance costs incurred during start-ups and shut-downs. However, operation at low loads results in lower operating efficiencies and higher operating costs.
Work from a gas turbine varies as a function of mass flow, heat energy in the combusted gas, and temperature differential across the turbine. These factors may be affected by ambient conditions, fuels, inlet and exhaust losses, fuel heating, diluent injection, air extraction, inlet cooling and steam and water injection. For example, changes in ambient conditions (pressure, temperature and humidity) affect the density and/or mass flow of the air intake to the compressor and consequently gas turbine performance. The mass flow is in turn a function of compressor airflow and fuel flow.
Compliance with emission standards is also a major constraint in the operation of gas turbines. Most gas turbines combust low sulfur and low ash fuels. Consequently, the major pollutants emitted from gas turbines are nitrogen oxides (NO and NO2, collectively referred to as NOx), carbon monoxide (CO), and volatile organic compounds (VOC). NOx and CO are considered the primary emissions of significance when combusting natural gas in gas turbines. Emissions from gas turbines vary significantly as a function of ambient temperature, load, and pollutant concentration. Below 50% load, emission concentrations may increase. This is especially true for carbon monoxide (CO). Consequently, there is a limit to the load level at which conventional gas turbine systems may be operated while still complying with emission standards.
Emission standards applicable to gas turbine operations may vary by country and in the United States, in addition to Federal standards, standards may vary from State to State. Regulatory authorities may impose various regimes for regulating emissions. For example, an NOx emission limit may be stated as pounds of NOx per unit of output, or per unit of heat input (instantaneous limit). In some cases the standards may be formulated as a concentration-based or an output-based emission standard. A concentration-based limit may be stated in units of parts per million by volume (ppmv). The output-based emission limit may be stated in units of emissions mass per unit useful recovered energy, or pounds per megawatt-hour. Some plants may be limited on the basis of the number of tons of NOx emitted per year or other time period (periodic limit).
Some power plants have emissions limits and other restrictions when in the startup mode. Types of startup limits include: (a) pounds per hour, (b) lb/event/CTG and (c) lb/event/power block. A maximum allowable lb/hour limit may be required by the regulatory agency, since it is the most straightforward value to use in an air quality impact assessment.
One of the problems with the operation of gas turbines over wide power ranges is that efficiency, fuel consumption and emissions, specifically, NOx and CO emissions, may be negatively affected. For example, when a plant operator operates a conventional gas turbine at low loads there is a significant decrease in efficiency. Another problem is that the compressor may be subjected to aeromechanical stresses in the aft stages when the gas-fired turbine is operated at lower loads in low ambient temperature conditions. These stresses occur below the aerodynamic stability limit due to an excitation of an aeromechanical mode which is driven by an increase in the stage loading parameter. The flow rate coefficient values at which these stresses are evident are referred to as the turndown restricted zone. Yet another problem with gas turbines operating at lower ambient temperatures is that the minimum load required for CO compliance is a function of, among other things, the ambient temperature. For example, in some gas turbines, as the temperature falls below 35 F (1.7 C), the minimum load for CO compliance rises steeply. Yet another problem is that the use of a heat recovery steam generator in a combined cycle gas turbine systems may impose additional constraints on the optimal gas turbine operation at base-load, part-load and load-ramp operating modes. Yet another problem is that when the gas turbine is operated at extreme low hibernation modes of approximately 10% load there is the potential of combustor lean blowout (i.e. loss of flame).